Lubricating oil is used in various kinds of machinery so as to improve lubricity between members. Increase of foaming of the lubricating oil may lead to insufficient lubrication, failure to control hydraulic pressure, deteriorated cooling efficiency, and so on. Thus, lubricating oil is required to suppress foaming.
For example, automobile engines, transmissions, and axle units are demanding increasingly more from lubricating oil, accompanying their recent improvements in performance and fuel efficiency. Continuous high-load operation or high-speed driving increases foaming in engine oil, transmission oil, or axle unit oil, and as a result, leads to problems such as: failure to control hydraulic pressure due to incorporation of foam in hydraulic circuits; deteriorated lubricating performance or cooling efficiency due to foaming; wear and seizure due to breakage of oil films in friction portions; and promoted deterioration of lubricating oil due to oil temperature increase. Thus, there is demand for engine oil, transmission oil, and axle unit oil which can maintain high defoaming performance so as to suppress foaming for a long time from the initial stage of use.
Generally, lubricating oil contains base oil, and various additives incorporated according to desired characteristics. Examples of additives include defoaming agents to prevent foaming of lubricating oil. Polysiloxane defoaming agents (silicone defoaming agents) are conventionally known as defoaming agents. For example, Patent Literature 1 describes a lubricating oil composition formed by incorporating (a) polydimethylsiloxane having kinematic viscosity at 25° C. of 300,000 to 1,500,000 mm2/s, and (b) fluorinated polysiloxane having kinematic viscosity at 25° C. of 500 to 9,000 mm2/s. Patent Literature 2 discloses incorporating polydimethylsiloxane having a specific molecular weight distribution into lubricating oil to obtain defoaming effect on foam generated due to high-speed stirring.